1. Field of the Invention
The present invention relates to an optical reading method for reading information by irradiating an optical recording medium with a laser beam and an optical reading system.
2. Description of the Related Art
Conventionally, optical recording media such as CD-DAs, CD-ROMs, CD-Rs, CD-RWs, DVD-ROMs, DVD-Rs, DVD±RWs, DVD-RAMs, and the like are widely used to watch digital moving image contents and record digital data. Conversely, the recording capacity required of these kinds of optical recording media grows with each passing year and the so-called next-generation optical discs, which can store massive amounts of both moving images and data, have come into commercial use to meet such a requirement. In the next-generation optical discs, the wavelength of a laser beam used for recording and reading is shortened to 405 nm in order to increase their recording capacity.
In the Blu-ray Disc (BD) standard, being one of the next-generation DVD standards, for example, the numerical aperture of an objective lens is set to 0.85 in order to record and read 25 GB of data on and from a single recording layer.
It should also be noted that it is expected that the amount of moving images and data will increase more and more in the future. Thus, in order to increase the capacity of an optical recording medium, a method for increasing the linear recording density of an information recording layer in the above described optical recording medium has been investigated. Also, as described in the articles of I. Ichimura et. al., Appl. Opt, 45, 1974-1803(2006) and K. Mishima et. al., Proc. of SPIE, 6282, 62820I(2006), a method of increasing the capacity of an optical recording medium by providing multiple information recording layers has been investigated.
In order to increase the linear recording density of the information recording layer, a recording mark has to be reduced to a small size. However, when λ represents the wavelength of a laser beam and NA represents the numerical aperture of an objective lens, if a sequence of a recording mark and a space each having a size of (λ/NA)/4 or less is contained in a particular encoding signal, the amplitude of the read signal from a train of recording marks and spaces becomes almost zero. Therefore, a so-called resolution limit exists.
According to a study undertaken by the inventors, although this study was publicly unknown at the time of filing of the present application, when the size of a minimum recording mark or a minimum space is 1.1×(λ/NA)/4 or less, the amplitude of a read signal from a train of the minimum recording marks and spaces becomes extremely small and a practical output cannot be obtained even with the use of an equalizer.
A PRML (partial response maximum likelihood) detection method is, however, known as one technology used for solving the problem of the resolution limit. In the PRML detection method, binary data recorded on an information recording layer are estimated on the basis of an electrical analog signal detected during optical reading. In the PRML detection method, the appropriate reference class characteristic (constraint length) of the PR (partial response) is selected in accordance with the reading characteristics. The constraint length needs to be determined in consideration with how much a laser beam spot for reading a target recording mark is susceptible to adjacent recording marks (optical interference), in other words, how much the state of adjacent recording marks/spaces add a constraint to the read signal output.
When T represents one unit clock period in a reading control system, the case where there is a train of recording marks/spaces with a length corresponding to 7 T in the beam spot is taken as an example. In this case, since the output value provided by the reflected light of the recording mark or space positioned in the center of the beam spot includes the reflected light of the train of recording marks/spaces with the length corresponding to 7 T (including the center), it is preferable that the constraint length n is set to be at least 5 or more.
In PRML detection with a constraint length of 5, it is assumed that when a sign bit “1” of the target recording mark is read, 5 bits in total including the adjacent sign bits are constrained (susceptible) As a result, a reading response waveform is equalized and decoded on the basis of being expressed by the result of the convolution of, for example, “12221.”
Accordingly, generally speaking, if the constraint length is set to be large, it becomes possible to include the optical interference of recording marks that are further away with respect to a recording mark, wherein the recording mark is the reading target in the calculation being undertaken. Taking the case where there is a train of recording marks/spaces with a length corresponding to 7 T in the beam spot as an example, setting the constraint length at 7 makes it possible to equalize an outputted waveform into a waveform the shape of which is close to the actual outputted waveform, and therefore decode it with great precision. However, there is a problem in that the scale of a calculation circuit exponentially increases with an increase in the constraint length. Therefore, by improving precision in decoding using the devised ML processing method, the constraint length can be set at 5, even in the case where there is a train of recording marks/spaces with a length corresponding to 7 T in the beam spot.
However, according to the unpublished study undertaken by the inventors, in the method used to increase the constraint length in order to increase recording capacity, which is based on the idea that precision increases with an increase in the constraint length (in principle), when an information recording layer is brought close to the light incident surface of an optical recording medium because of multiple layers or the like, the PRML detection processing does not function efficiently due to the existence of extraneous matters such as fingerprints and the like and reading quality decreases.